Download 17.1 Genes and Variation

Lesson Overview
17.1 Genes and Variation
Lesson Overview
Genes and Variation
THINK ABOUT IT
Darwin developed his theory of evolution without knowing how heritable
traits passed from one generation to the next or where heritable variation
came from.
What would happen when genetics answered questions about how heredity
works?
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Genes and Variation
Genetics Joins Evolutionary Theory
How is evolution defined in genetic terms?
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Genes and Variation
Genetics Joins Evolutionary Theory
How is evolution defined in genetic terms?
In genetic terms, evolution is any change in the relative frequency of alleles in the
gene pool of a population over time.
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Genes and Variation
Genetics Joins Evolutionary Theory
Researchers discovered that heritable traits are controlled by genes.
Changes in genes and chromosomes generate variation.
For example, all of these children received their genes from the same parents, but
they all look different.
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Genes and Variation
Genotype and Phenotype in Evolution
An organism’s genotype is the particular combination of alleles it carries.
An individual’s genotype, together with environmental conditions, produces its
phenotype.
Phenotype includes all physical, physiological, and behavioral characteristics of an
organism.
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Genes and Variation
Genotype and Phenotype in Evolution
Natural selection acts directly on phenotype, not genotype.
Some individuals have phenotypes that are better suited to their environment
than others. These individuals produce more offspring and pass on more copies of
their genes to the next generation.
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Genes and Variation
Populations and Gene Pools
A population is a group of individuals of the same species that mate and produce
offspring.
A gene pool consists of all the genes, including all the different alleles for each
gene that are present in a population.
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Genes and Variation
Populations and Gene Pools
Researchers study gene pools by examining the relative frequency of an allele.
The relative frequency of an allele is the number of times a particular allele occurs
in a gene pool, compared with the number of times other alleles for the same
gene occur.
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Genes and Variation
For example, this diagram shows the gene pool for fur color in a population of
mice.
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Genes and Variation
Populations and Gene Pools
Evolution is any change in the relative frequency of alleles in the gene pool of a
population over time.
Natural selection operates on individuals, but resulting changes in allele
frequencies show up in populations. Populations, rather than individuals, evolve.
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Genes and Variation
Sources of Genetic Variation
What are the sources of genetic variation?
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Genes and Variation
Sources of Genetic Variation
What are the sources of genetic variation?
Three sources of genetic variation are mutation, genetic recombination during
sexual reproduction, and lateral gene transfer.
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Genes and Variation
Mutations
Mutations that produce changes in phenotype may or may not affect fitness.
Some mutations may be lethal or may lower fitness; others may be beneficial.
Mutations matter in evolution only if they can be passed from generation to
generation. The mutation must occur in the germ line cells that produce either
eggs or sperm.
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Genes and Variation
Genetic Recombination in Sexual Reproduction
Mutations that produce changes in phenotype may or may not affect fitness. Some
mutations may be lethal or may lower fitness; others may be beneficial.
Mutations matter in evolution only if they can be passed from generation to
generation. The mutation must occur in the germ line cells that produce either eggs or
sperm.
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Genes and Variation
Lateral Gene Transfer
Lateral gene transfer occurs when organisms pass genes from one individual to
another that is not its offspring.
It can occur between organisms of the same species or organisms of different
species.
Lateral gene transfer can increase genetic variation in a species that picks up the
“new” genes.
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Genes and Variation
Single-Gene and Polygenic Traits
What determines the number of phenotypes for a given trait?
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Genes and Variation
Single-Gene and Polygenic Traits
What determines the number of phenotypes for a given trait?
The number of phenotypes produced for a trait depends on how many genes
control the trait.
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Genes and Variation
Single-Gene Traits
A single-gene trait is a trait controlled by only one gene. Single-gene traits may
have just two or three distinct phenotypes.
The most common form of the allele can be dominant or recessive.
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Genes and Variation
Dominance of an allele for a single-gene trait does not necessarily mean that the
dominant phenotype will always appear with greater frequency in a given
population. An example of a single-gene trait is the presence of dark bands that
appear on the shells of a certain species of snails. Even though the allele for shells
without bands is dominant, a population may show a greater frequency of the
“with bands” phenotype.
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Genes and Variation
Polygenic Traits
Polygenic traits are traits controlled by two or more genes.
Each gene of a polygenic trait often has two or more alleles.
A single polygenic trait often has many possible genotypes and even more
different phenotypes.
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Genes and Variation
Polygenic Traits
Human height, which varies from very short to very tall, is an example of a
polygenic trait.
The bell-shaped curve in the graph is typical of polygenic traits.